Simplified API

I've started simplifying the model-level CRUD API. As a first, most models have a `.get` classmethod that retrieves an instance using the most common data columns (usernames and userids for User, tokens for AuthToken, etc). The `get` method ensures queries apply constraints such as checking that a User is active.

I will be adding `get` methods to all models and replacing all direct access to `model.query` in forms and views. Currently all forms have been migrated, views are pending.